Electronic circuit for an energy supply device, especially for a charging device for batteries

ABSTRACT

The invention relates to an electronic circuit for a power supply device (1), in particular for a charger (2) for accumulators (3), in which the power supply device (1) is connected via input terminals (4, 5) to a power source (6), the power supply device (1) converting the supplied power from any voltage, in particular an AC voltage, into a DC voltage and this transformed power is forwarded via output terminals (7, 8) to a consumer (9), in particular the accumulator (3), in which, a switching element (10) is provided as a means of operating polarity reversal protection. The switching element (10) is disposed between at least one output terminal (7, 8) and a potential, in particular a negative potential, of the power supply device (1) and if a specific polarity appears at this output terminal (7, 8) the switching element (10) is connected through and if an opposite (incorrect) polarity appears it is switched off. The output terminals (7, 8) are disconnected separated from the input terminals (4, 5) , in particular from the power source (6), by the power supply device (1).

[0001] The invention relates to an electronic circuit for an energy supply device, and a method of controlling an electronic circuit for a power supply device, as outlined in claims 1 and 11.

[0002] Patent specification DE 197 56 825 A1 describes a polarity reversal protection circuit with a switching element disposed between an input terminal and an output terminal, which does not allow a current flow unless the voltage at the input terminal is at a certain polarity. If a specific polarity is applied to the input terminal, the switching element connects through and if the polarity is not correct, it is opened or deactivated. The disadvantage of this system is that there is protection on the input side only.

[0003] An electronic circuit with polarity reversal protection is also known from patent specification DE 197 17 527 A1, in which a switching element, in particular a MOS field-effect transistor, is connected between an input source connected to a DC voltage source and a downstream circuit structure, and the switching element is activated if the DC voltage source is at the correct polarity and deactivated if the polarity is not correct. Again, the disadvantage of this approach is that there is protection on the input side only.

[0004] The underlying objective of the invention is to propose an electronic circuit for a power supply device and a method of controlling an electronic circuit for a power supply device, which will protect a power supply device on the output side, such as a polarity reversal protection, an overload protection and/or a load detection system, for example.

[0005] This objective is achieved by the invention due to the fact that the switching element for the consumer is disposed between at least one output terminal and a potential, in particular a negative potential, of the power supply device for the consumer and if a specific polarity is applied to this output terminal, the switching element is switched on and if the opposite (incorrect) polarity is applied, it is switched off, in which case the output terminals are disconnected from the input terminals of the power supply device, in particular the power source, by the power supply device. The advantage of this approach is that a circuit design of this type for charging an accumulator does not require a back voltage, which enables fully discharged accumulators to be charged. Another advantage resides in the fact that not only can a power supply device of this type be used to charge accumulators, it can also be used to supply power to other consumers because, apart from providing polarity reversal protection, it also offers other safety features, such as overload protection and load detection.

[0006] Other features are defined in claims 2 to 10. The resultant advantages may be found in the description.

[0007] The objective is also achieved by the invention as a result of the features defined in claim 11, whereby when the power supply device is activated, in other words when it is connected to the power source, the switching element is activated between an output terminal and the preferably negative potential of a voltage, in particular a supply voltage or an output voltage of the power supply device, establishing an electrical connection between this output terminal and the preferably negative potential, and if a specific polarity is applied to this output terminal, in particular a negative polarity, the switching element remains switched on, whereas if the opposite (incorrect) polarity is applied to this output terminal, in particular a positive potential, the switching element is switched off or deactivated. The advantage of this approach is that regardless of whether a consumer is connected, the power supply device is fully protected on the output side.

[0008] Other features are defined in claims 12 to 19. The resultant advantages may be found in the description.

[0009] The invention will be explained in more detail with reference to examples of embodiments.

[0010] Of the drawings:

[0011]FIG. 1 is a simplified diagram illustrating a circuit design of an electronic circuit for a power supply device, as proposed by the invention;

[0012]FIG. 2 is a simplified diagram illustrating another circuit design of an electronic circuit for a power supply device, as proposed by the invention.

[0013] Firstly, it should be pointed out that the same parts and states in the individual embodiments described as examples are denoted by the same reference numbers.

[0014]FIGS. 1 and 2 illustrate circuit layouts of an electronic circuit for a power supply device 1, in particular for a charger device 2 for accumulators 3. The power supply device 1 or charger 2 is symbolically illustrated by a simple block and the circuit design may be any system known from the prior art. For example, the power supply device 1 or charger 2 might be used as an inverter source or similar. Basically, it should be pointed out that the circuit design illustrated in detail in FIGS. 1 and 2 is disposed or integrated in the power supply device 1 or charger device 2.

[0015] In the electronic circuit for the power supply device 1, in particular the charger 2 for accumulators 3, the power supply device 1 is connected to a power source 6, in particular a mains network, via input terminals 4, 5. The power supply device 1 converts the input energy from any voltage, in particular an AC voltage, into a DC voltage and forwards this converted power, in particular the DC voltage, via output terminals 7, 8 to a consumer 9, so that the consumer 9, in particular an ohmic consumer 9, can be supplied with power, as indicated by broken lines, and an accumulator 3 can be charged.

[0016] It should also be pointed out that by means of the electronic circuit, which is outlined by dotted-dashed lines, the power supply device 1 is able to run several functions, in particular polarity reversal protection, overload protection and/or load detection. The individual functions will be described in more detail below. In the drawings, which will be explained below, the electronic circuit is designed for a specific potential, in particular the negative potential, although it would also be possible to operate on the basis of a different potential, in particular a positive potential.

[0017] As illustrated in FIG. 1, a switching element 10 is provided between at least one output terminal 7 or 8, in particular the output terminal 8 for the negative potential, and a specifically negative potential of the power supply device 1 for the consumer 9, and if a specific polarity, in particular a negative polarity, is applied to this output terminal 8, the switching element 10 is switched on and if the opposite (incorrect) polarity, in other words positive polarity, appears, it is switched off, the switching function of the electronic circuit and/or control device 11 of the power supply device 1 being operated automatically, as will be described below. The output terminals 7, 8 are disconnected from the input-terminals 4, 5 of the power supply device 1, in particular the power source 6, by means of the power supply device 1, so that the individual possible functions, in particular polarity reversal protection, overload protection and/or load detection, operate by reference to the output region.

[0018] Connected to the output terminal 8 for the preferably negative potential is a terminal 12, in particular a drain terminal, of the switching element 10, in particular a MOS field-effect transistor. Another terminal 13, in particular a source terminal, of the switching element 10 is connected to the negative potential. The through connection, in particular the drain-source connection, of the switching element 10 is therefore connected between the output terminal 8 and the potential, in particular the negative potential, of the power supply device 1 and if a specific polarity appears at the output terminal 8, the through connection is switched on whereas if the opposite (incorrect) polarity appears at the output terminal 8, the through connection is switched off.

[0019] A control terminal 14, in particular a gate terminal, of the switching element 10 is connected to a voltage, in particular a positive supply voltage 16, preferably via a resistor 15, as schematically illustrated. This connection may be set up so that the control terminal 14 is connected to an output voltage of the power supply device 1, for example, in particular the output terminal 7, or a voltage of an independent power source. The key factor is that the control terminal 14 is powered or supplied by a permanent voltage which is not interrupted by a switch element, i.e. the control terminal 14 of the switching element 10 is continuously supplied with voltage, preferably a positive voltage. As a result, when this voltage is applied to the control terminal 14, the switching element 10 is automatically activated, i.e. when this voltage, in particular the supply voltage 16, is applied or activated, the switching element 10 is activated without a consumer 9 having to be connected to the output terminals 7, 8.

[0020] The control terminal 14, in particular the gate terminal, of the switching element 10 is connected to a terminal 17, in particular a collector terminal, of another switching element 18, in particular a transistor. Another terminal 19, in particular an emitter terminal, of the second switching element 18 is connected to the negative potential. The through connection, in particular the collector-emitter connection, of the other switching element 18 is therefore disposed between the control terminal 14, in particular the gate terminal, of the first switching element 10, and the preferably negative potential.

[0021] The terminal 12, in particular the drain terminal, of the first switching element 10 is connected to the output terminal 8 and preferably via a resistor 20 to a control terminal 21, in particular a base terminal, of the second switching element 18 so that a corresponding voltage can be applied in order to activate the second switching element 18 from the output terminal 8 on the control terminal 21 of the other switching element 18. The control terminal 21, in particular the base terminal, is also connected to the control device 11 of the power supply device 1, in particular to an output, the control device 11 being schematically indicated by a block and preferably being provided in the form of a microprocessor. The control terminal 14, in particular the gate terminal, of the first switching element 10 or the terminal 17, in particular the collector terminal, of the second switching element 18 is also connected to the control device 11, in particular to an input of the control device 11.

[0022] The essential feature of the electronic circuit proposed by the invention is that the first switching element 10 can also be activated when the power supply device 1 is switched on, even if no consumer 9 is connected, in particular the accumulator 3. This is possible because the control terminal 14 of the first switching element 10 is permanently supplied with voltage so that once this voltage has built up it is automatically able to activate the first switching element 10.

[0023] As a result of this build-up, the switching element 10 is activated with a specific polarity at the output terminals 7, 8 when the consumer 9 is connected, even if no power source 6 is connected to the input terminals 4, 5 or the voltage applied to the gate terminal is activated when the control terminal 14 of the first switching element 10 is connected to the output voltage, in particular the other output terminal 7, instead of the supply voltage 16 because this causes a corresponding accumulator voltage to appear at the control terminal 14 of the first switching element 10. Consequently, the electronic circuit, in particular the polarity reversal protection, is active even without a power source 6 at the input terminals 4, 5, thereby preventing damage to the components if the accumulator 3 is incorrectly connected to the output terminals 7, 8 when the power supply device 1 is not in operation.

[0024] The operating principle and the method by which the electronic circuit for the power supply device 1, in particular the charger 2 for accumulators 3, is run will be described in more detail below. As explained above, the power supply device 1 is connected via input terminals 4, 5 to the power source 6, so that the power supply device 1 converts the supplied power from any voltage, in particular an AC voltage, into a DC voltage and this converted power is forwarded via output terminals 7, 8, which are separated from the input terminals 4, 5 by components such as diodes or a transformer, etc., to the consumer 9, in particular the accumulator 3. When the power supply device 1 is activated, in other words when connected to the power supply 6 or by switching on the power supply device 1, the switching element 10 between the output terminal 8 and the preferably negative potential of a voltage, in particular the supply voltage 16 or an output voltage of the power supply device 1 or an independent voltage is activated, establishing an electrical connection between this output terminal 8 and the preferably negative potential. As a result of establishing a connection between the output terminal 8 and a potential, in particular the negative potential, the functions mentioned above can now be run, in particular the polarity reversal protection, the overload protection and/or the load detection, because a current circuit can be established by the power supply device 1 across the two output terminals 7, 8.

[0025] The polarity reversal protection is operated in such a way that if an external voltage source is applied to the output terminals 7, 8 with a specific, in particular negative, polarity at the output terminal 8, for example by connecting the accumulator 3, the switching element 10 remains switched on whereas if the external voltage source, in particular the accumulator 3, is connected to the output terminal with the opposite, in particular positive polarity, the switching element 10 is switched off or deactivated. The way in which this operates is that if the output terminals 7 and 8 are at the opposite (incorrect) polarity, the other, in particular second, switching element 18 is activated so that the voltage appearing at or forwarded to the control terminal 14 of the first switching element 10, in particular the supply voltage 16, is applied to the negative potential and the first switching element 10 is therefore locked or deactivated, i.e. because the polarity at the output terminal 8 is opposite (incorrect), in this case positive, the voltage applied via the connecting line between the output terminal 8 and the control terminal 21 of the second switching element 18 is forwarded to the latter so that the other switching element 18 is activated or connected and the first switching element 10 is deactivated by the connection established between the control terminal 14 and the negative potential.

[0026] The first switching element 10 remains deactivated until the opposite (incorrect) polarity of the foreign voltage of the accumulator 3 at the output terminals 7, 8 is interrupted so that the supply voltage at the control terminal 21 of the other switching element 18 is also interrupted, thereby locking or deactivating the other switching element 18 again and the voltage, in particular the supply voltage 16, at the control terminal 14 of the first switching element 10 can be built up again enabling the first switching element 10 to be automatically activated.

[0027] Basically, it should be pointed out that it is not just accumulators 3 which can be connected to the output terminals 7, 8. Naturally, it would also be possible to connect an ohmic resistance or other circuit components which need to be supplied with power from the power supply device 1. However, if no accumulator 3 or no foreign voltage source is connected, it will not be possible to operate polarity reversal protection and the electronic circuit will then only be able to run the overload protection and/or load detection functions.

[0028] The overload protection prevents too high a flow of current across the first switching element 10, which could damage the switching element 10. In the case of an accumulator 3, too high a flow of current might occur if the latter were totally or almost totally discharged.

[0029] In the case of the overload protection, in other words when connecting an empty or almost empty accumulator 3 or another consumer 9 with a high current uptake to the output terminals 7, 8, the first switching element 10 is deactivated by the second switching element 18, even if the consumer 9 or the accumulator 3 is at a specific (correct) polarity. The way this is operated is that, as a result of too high a current flow, a corresponding voltage is appears via the resistor 20 at the control terminal 21 of the other switching element 18 so that when a specific level is exceeded, the other switching element 18 is activated, thereby deactivating the first switching element 10 in the manner described above with respect to the polarity reversal protection. The permissible current flow across the first switching element 10 can be defined on the basis of the rating of the resistor 20 in the connecting line to the control terminal 21 of the second switching element 18 and the rating of the switching element 10.

[0030] When the overload protection is in operation when an accumulator 3 is being used as the consumer 9, the first switching element 10 is periodically switched on and off until the current flow being established across the first switching element 10 falls below a defined value and the voltage appearing at the control terminal 21 of the other switching element 18 also falls below a pre-defined value, preventing the other switching element 18 from being activated. The switching element 10 is periodically switched on and off because whenever the first switching element 10 is activated, the empty or almost empty accumulator 3 connected to it is charged for a brief period, thereby reducing the current level over time and enabling permanent charging.

[0031] In order to enable this periodic switching on and off, however, the control device 11 must intervene accordingly, i.e. the second switching element 18 must be purposely deactivated by the control device 11 so that the first switching element 10 is activated again. To do this, however, it is necessary for the control device 11 to detect the state of the first and second switching elements 10 and 18. This is achieved due to the fact that a control input 22 of the switching device 11 is preferably coupled with the control terminal 14 of the first switching element 10 and with the collector terminal of the second switching element 18 via a resistor 23, so that when the second switching element 18 is activated, the voltage appearing at the control terminal 14 of the first switching element 10, in particular the supply voltage 16, is applied to the preferably negative potential, thereby enabling a high or low signal to occur or be generated at the control input 22 of the control device 11. As a result, the control device 11 will be able to detect the state of the switching elements 10 and 18 for all functions, in other words for the polarity reversal protection and/or load detection. With an electronic circuit design of this type, the two switched states of the switching elements 10 and 18 can be detected on the basis of only one signal, in particular a high or low signal, since the two switching elements 10 and 18 are always in a complementary switched state, i.e. when the second switching element 18 is activated, the first switching element 10 is deactivated and vice versa.

[0032] To enable the control device 11 to activate the second switching element 18, the control device 11 has a control output 24 connected to the control terminal 21 of the second switching element 18, preferably via a resistor 25. By means of this connection, the control device 11 can intervene in the electronic circuit for control purposes and can control the second switching element 18 by applying a signal to the control terminal 21. This can be operated in such a way, for example, that when the first switching element 10 is deactivated by the control device 11 of the power supply device 1 on expiry of a pre-settable period, the second activated switching element 18 is deactivated by transmitting a negative signal, so that the voltage applied to the control terminal 14 of the first switching element 10 activates the first switching element 10 again. This switching sequence can be repeated by the control device 11 any number of times to enable the first switching element 10 to be periodically switched on and off. Naturally, the system could be set up so that after a pre-defined number of switching sequences the control device 11 does not initiate a further switching sequence and an error signal is output.

[0033] In order to be able to operate a load detection system, the control device 11 automatically controls the switching elements 10 and 18, in particular the second switching element 18. For load detection purposes, the control device 11 firstly deactivates the first switching element 10 by activating the second switching element 18 and, on expiry of a pre-settable period, activation of the second switching element 18 is terminated so that if a consumer 9 is connected, a voltage will appear at the control terminal 21 of the second switching element 18, in which case the first switching element 10 will remain deactivated on the basis of the self-holding function due to the voltage appearing at the control terminal 21. This is detected by the control device 11 via the control input 22 and it will be concluded from this that a consumer 9 is connected. By adopting this approach, it is possible to connect an ohmic resistor or other circuits groups or devices as consumers 9.

[0034] To enable the control device 11 to detect an accumulator 3 at the output terminals 7, 8, another control procedure is necessary in addition to that described above. This is necessary because when an accumulator 3 is connected at the same voltage level as the output voltage of the power supply device 1, no voltage is registered across the resistor 20 at the control terminal 21 of the other switching element 18 and it is not possible to operate a self-holding function.

[0035] To operate a load detection system of this type, in particular the detection of an accumulator 3, whilst the first switching element 10 is deactivated, the control device 11 raises the output voltage at the output terminals 7, 8 from the power supply device 1 by a certain amount, in particular between 1V and 5V, causing the control device 11 to terminate activation of the second switching element 18, as a result of which, when an accumulator 3 is connected, voltage is applied to or registered at the control terminal 21 of the second switching element 18 because of the voltage difference between the accumulator voltage and the output voltage of the power supply device 1 and the second switching element 18 will therefore remain active because of the self-holding function, as described above, and this will in turn be detected by the control device 11 via the control input 22.

[0036] It should be pointed out in respect of the operating principles described above that the polarity reversal protection can be operated independently of any intervention from the control device 11, whereas in the case of the overload protection and/or load detection, the control device 11 must intervene accordingly, so that these functions are run or repeated during operation respectively after the power supply device 1 has been switched on and/or on expiry of any set period.

[0037]FIG. 2 illustrates another design of the electronic circuit, the operating principles and control sequences being run in the same way as those described with reference to FIG. 1.

[0038] The difference compared with the design illustrated in FIG. 1 is that the control terminal 14 of the first switching element 10 in this case is directly connected to a control output 26 of the control device 11, so that the connecting line illustrated in FIG. 1 to the other switching element 18 and hence to the supply voltage 16 or to the output voltage of the power supply device 1 is interrupted.

[0039] To enable the functions, in particular the polarity reversal protection, the overload protection and/or the load detection system, to be run by the power supply device 1, the switching element 10 will have to be controlled from the control device 11 , i.e. in accordance with the operating sequence described with regard to FIG. 1, the control device deactivates the switching element 10 when the switching element 18 is activated and vice versa. Such a control system can be operated via the control device 11 because the control device 11 is able to detect the state of the second switching element 18 via the control input 22 and the first switching element 10 can be controlled accordingly by the control device 11.

[0040] For the sake of good order, it should be pointed out that in order to provide a clearer understanding of the power supply device 1, it and its constituent parts are illustrated to a certain extent out of scale and/or on an enlarged scale and/or on a reduced scale.

[0041] The objectives underlying the solutions proposed by the invention may be found in the description.

[0042] Above all, the individual embodiments and features illustrated in FIGS. 1 and 2 may be construed as independent solutions proposed by the invention. The related objectives and solutions proposed by the invention may be found in the detailed description of the drawings. List of reference numbers 1 Power supply device 2 Charging device 3 Accumulator 4 Input terminal 5 Input terminal 6 Power source 7 Output terminal 8 Output terminal 9 Consumer 10 Switching element 11 Control device 12 Terminal 13 Terminal 14 Control terminal 15 Resistor 16 Supply voltage 17 Terminal 18 Switching element 19 Terminal 20 Resistor 21 Control terminal 22 Control input 23 Resistor 24 Control output 25 Resistor 26 Control output 

1. Electronic circuit for a power supply device (1), in particular for a charger (2) for accumulators (3), in which the power supply device (1) is connected via input terminals (4, 5) to a power source (6), the power supply device (1) converting the supplied power from any voltage, in particular an AC voltage, into a DC voltage and this converted power is forwarded via output terminals (7, 8) to a consumer (9), in particular the accumulator (3), in which, a switching element (10) is provided as a means of operating polarity reversal protection, characterised in that the switching element (10) is disposed between at least one output terminal (7, 8) and a potential, in particular a negative potential, of the power supply device (1), and if a specific polarity appears at this output terminal (7, 8) the switching element (10) is connected through and if an opposite (incorrect) polarity appears it is switched off, at least one of the output terminals (7, 8) being disconnected from the input terminals (4, 5) , in particular from the power source (6), by the power supply device (1).
 2. Electronic circuit as claimed in claim 1, characterised in that the output terminal (7, 8) for the preferably negative potential is connected to a terminal (12), in particular a drain terminal, of the switching element (10), in particular a MOS field-effect transistor, the through connection, in particular a drain-source through connection, is connected between the output terminal (7, 8) and the specifically negative potential of the power supply device (1) and if a specific polarity appears at the output terminal (7, 8) the through connection is switched through, whereas if the opposite (incorrect) polarity appears at the output terminal (7, 8), the through connection is interrupted.
 3. Electronic circuit as claimed in claim 1 or 2, characterised in that a control terminal (14), in particular a gate terminal, of the switching element (10) is connected, preferably via a resistor (15), to a supply voltage (16), in particular positive, or an output voltage of the power supply device (1) or a voltage of an independent power source.
 4. Electronic circuit as claimed in one or more of the preceding claims, characterised in that the control terminal (14), in particular the gate terminal, of the switching element (10) is additionally connected to a terminal (17), in particular a collector terminal, of another switching element (18), in particular a transistor.
 5. Electronic circuit as claimed in one or more of the preceding claims, characterised in that the through connection, in particular a collector-emitter connection, of the other switching element (18), in particular the transistor, is disposed between the control terminal (14), in particular the gate terminal, of the first switching element (10), and the preferably negative potential.
 6. Electronic circuit as claimed in one or more of the preceding claims, characterised in that a terminal (12), in particular the drain terminal, of the first switching element (10) is connected to the output terminal (7, 8) and preferably via a resistor (20) to a control terminal (21), in particular a base terminal, of the second switching element (18).
 7. Electronic circuit as claimed in one or more of the preceding claims, characterised in that the control terminal (21), in particular the base terminal, is connected to a control device (11) of the power supply device (1).
 8. Electronic circuit as claimed in one or more of the preceding claims, characterised in that the control terminal (14), in particular the gate terminal, of the first switching element (10) and/or a terminal (17), in particular a collector terminal, of the second switching element (18) is connected to the control device (11).
 9. Electronic circuit as claimed in one or more of the preceding claims, characterised in that the first switching element (10) is activated when the power supply device (1) is switched on, even if no consumer (9) is connected.
 10. Electronic circuit as claimed in one or more of the preceding claims, characterised in that the switching element (10) is activated at a specific polarity when a consumer (9), in particular the accumulator (3) is connected to the output terminals (7, 8) , even if no power source (6) is connected to the input terminals (4, 5).
 11. Method of controlling an electronic circuit for a power supply device (1), in particular for a charger (2) for accumulators (3), in which the power supply device (1) is connected via input terminals (4, 5) to a power source (6), the power supply device (1) converting the supplied power from any voltage, in particular an AC voltage, into a DC voltage and this transformed power is forwarded via output terminals (7, 8) to a consumer (9), in particular the accumulator (3), in which a switching element (10) is provided as a means of operating polarity reversal protection, characterised in that when the power supply device (1) is activated, in other words when it is connected to the power source (6), the switching element (10, 18) which is disposed between an output terminal (7, 8) and the preferably negative potential of a voltage, in particular the supply voltage (16) or an output voltage of the power supply device (1), is activated so that an electrical connection is established between this output terminal (7, 8) and the preferably negative potential of the supply voltage (16) or output voltage, and if a specific polarity, in particular a negative polarity, is applied to the consumer (9), in particular the accumulator (3), at this output terminal (7, 8), the switching element (10) remains connected through, whereas if an opposite (incorrect) polarity is applied to the consumer (9), in particular the accumulator (3), at this output terminal (7, 8), the switching element (10) is switched off or deactivated.
 12. Method as claimed in claim 11, characterised in that if an opposite (incorrect) polarity appears at the output terminals (7, 8), another, in particular a second switching element (18) is activated, as a result of which the voltage at a control terminal (14) of the first switching element (10) is forwarded to the negative potential and the first switching element (10) is therefore locked or deactivated.
 13. Method as claimed in claim 11 or 12, characterised in that a polarity reversal protection, an overload protection and a load detection system is run by the power supply device (1).
 14. Method as claimed in one or more of the preceding claims, characterised in that for the purpose of the overload protection system, when the polarity of the consumer (9) is at the specific (correct) polarity and too high a current flow is passing across the first switching element (10), the latter is deactivated by the second switching element (18).
 15. Method as claimed in one or more of the preceding claims, characterised in that in order to operate the overload protection, the first switching element (10) is periodically switched on and off until the current flow being established across the first switching element (10) falls below a defined value and the voltage appearing at a control terminal (21) of the other switching element (18) also falls below a pre-defined value, preventing the other switching element (18) from being activated.
 16. Method as claimed in one or more of the preceding claims, characterised in that when the first switching element (10) is deactivated on expiry of a pre-settable time by a control device (11) of the power supply device (1), the other activated switching element (18) is deactivated and the first switching element (10) is automatically re-activated by the applied voltage.
 17. Method as claimed in one or more of the preceding claims, characterised in that for the purpose of the load detection system, the control device (11) firstly deactivates the first switching element (10) by activating the second switching element (18) and on expiry of a pre-settable time, activation of the second switching element (18) is terminated, and if a consumer (9) is connected, a voltage is applied to the control terminal (21) of the second switching element (18), as a result of which the first switching element (10) remains deactivated due to the self-holding function and this is detected by the control device (11).
 18. Method as claimed in one or more of the preceding claims, characterised in that for the purpose of the load detection system, in particular for detecting the accumulator (3), whilst the first switching element (10) is deactivated, the output voltage at the output terminals (7, 8) is increased by the power supply device (1) by a specific amount, in particular 1V to 5V, and the control device (11) terminates activation of the second switching element (18) so that if a consumer (9), in particular an accumulator (3) is connected, a voltage appears at the control input (22) of the second switching element (18) and the second switching element (18) therefore remains active on the basis of the self-holding function, and this is detected by the control device (11).
 19. Method as claimed in one or more of the preceding claims, characterised in that the control device (11) detects that the first switching element (10) has been deactivated due to the fact that a control input (22) of the switching device (11) is coupled with a control terminal (21) of the first switching element (10) and, as a result, when the second switching element (18) is activated, the voltage appearing at the control terminal (21 ) of the first switching element (10) is applied to the preferably negative potential. 